This depends on the specs you want, since there's a pretty big gap between the 820 and 506. The Sony will not only accept Linear PCM (from a PS3, for instance) but it will also decode HD formats like DTHD and DTSHD-MA onboard. The Onkyo will do neither.
So it really depends on what you want - if you're only after basic surround, the Onkyo will serve you well. But for similar money, the Sony will go one step further and give you HD audio. To me, it's a no-brainer - wait for the 820!